VISHAY SEMICONDUCTORS
Optocouplers and Solid-State Relays
Application Note
Thermal Characteristics of Power Phototriac
INTRODUCTION
Behavior of any semiconductor device depends on the
temperature of its die. Hence, electrical parameters are
given at a specified temperature. To sustain the performance
of a component and to avoid failure or damage caused by
overheat, the total power dissipation of the device has to be
limited according to the maximum operating temperature.
Vishay VO3526 is a three dice DIP-16 power phototriac. Its
detailed thermal model was simulated and validated against
experimental results. It may help understanding the thermal
energy transferring, assisting power dissipation calculation
and PCB layout. For a detailed explanation of the thermal
model principle, please refer to the Vishay’s application note
“Thermal Characteristics of Optocouplers”.
THERMAL ENERGY TRANSFER
There are three mechanisms by which thermal energy (heat)
is transported; conduction, radiation, and convection.
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Fig. 1 - Example of Package Temperature Profile (2-Layer Board)
• Heat conduction is the transfer of heat from warm areas to
cooler ones, and effectively occurs by diffusion.
• Heat radiation (as opposed to particle radiation) is the
transfer of internal energy in the form of electromagnetic
waves.
• Heat convection is the transfer of heat from a solid surface
to a moving liquid or gas.
Document Number: 81165
Rev. 1.0, 03-Aug-09
21826
Fig. 2 - Example of Package Temperature Profile (4-Layer Board)
For technical questions, contact: optocoupleranswers@vishay.com
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297
APPLICATION NOTE
All three methods occur in phototriac, however in most
environments, the majority (~ 70 %) of heat leaving the
package exits through the lead frame and into the board.
This occurs because the heat transfer from board to ambient
is a conductive phenomenon with a much lower thermal
resistance (θBA) than the convective and radiative
phenomena associated with θCA (case to ambient thermal
resistance, typically an order of magnitude larger than other
thermal resistances when the device package is small).
This is shown graphically by the package temperature profile
and strong heat flux contours evident in the die, lead frame,
and board in figures 1 (2-layer board, higher θBA), and
2 (4-layer board, lower θBA). θBA is the thermal resistance
from the board to the ambient, and is primarily driven by the
geometry and composition of the board. The type of board
design used defines this characteristic.
Application Note
Vishay Semiconductors
Thermal Characteristics of Power Phototriac
POWER PHOTOTRIAC THERMAL MODEL
TA:
Ambient temperature
Vishay power phototriac VO3526 is a device with three dice,
emitter, opto-triac and non-opto-triac (power triac). Its
resistor network was derived from the experimental and
validated model results and is outlined below.
QE:
Power dissipation on emitter
TCASE
TCASE
TB
TB
θNOT-C
θNOT-B
θOT-C
QOT
TNOT
TOT
θE-NOT
Power Triac
Thermal Model
θE-OT
TB
QE
TCASE
TE
θE-B
θE-C
θBA
TB
θCA
TCASE
21295
TA
Fig. 3 - Three Dice Power Phototriac
Thermal Model/Resistor Network
Where:
E:
Emitter
OT:
Opto-triac
NOT:
Non-opto-triac
θE-B:
Thermal resistance, emitter die to board
θOT-B:
Thermal resistance, OT die to board
θNOT-B: Thermal resistance, NOT die to board
θEOT:
Power dissipation on OT
Power dissipation on NOT
In order to write equation to calculate the node temperatures,
some heat flow directions are assumed and shown on figure 3.
Based on figure 3, the following equations in table 1 are
created for calculating the node temperatures.
TABLE 1 - NODAL EQUATIONS
(For Reference)
θOT-B
θNOT-OT
QNOT
QOT:
QNOT:
Thermal resistance, emitter die to OT die
1
QNOT-OT + QNOT-E + QNOT-B + QNOT-C = QNOT
2
QOT-E - QNOT-OT + QOT-B + QOT-C = QOT
3
- QNOT-E - QOT-E + QE-B + QE-C = QE
4
TNOT - TOT - θNOT-OTQNOT-OT = 0
5
TNOT - TE - θNOT-EQNOT-E = 0
6
TNOT - TB - θNOT-BQNOT-B = 0
7
TNOT - TC - θNOT-CQNOT-C = 0
8
TOT - TB - θOT-BQOT-B = 0
9
TOT - TC - θOT-CQOT-C = 0
10
TOT - TE - θOT-EQOT-E = 0
11
TE - TB - θE-BQE-B = 0
12
TE - TC - θE-CQE-C = 0
13
TB - θBA(QNOT-B + QOT-B + QE-B) = TA
14
TC - θCA(QNOT-C + QOT-C + QE-C) = TA
Assuming the power dissipated from the triacs (QNOT and
QOT) and the emitter (QE), the temperatures of board (TB),
case (TC) and ambient (TA), are known (by measuring or
estimated), the node temperatures can be calculated by
EXCEL tools/solver or solving the network equations using
matrix calculation.
A*X=B
θE-NOT: Thermal resistance, emitter die to NOT die
APPLICATION NOTE
θNOT-OT: Thermal resistance, NOT die to OT die
θE-C:
Thermal resistance, emitter die to case
θO-TC:
Thermal resistance, OT die to case
θNOT-C: Thermal resistance, NOT die to case
θBA:
Thermal resistance, board to ambient
θCA:
Thermal resistance, case to ambient
TE:
Emitter junction temperature
TOT:
OT junction temperature
TNOT:
NOT junction temperature
TC:
Case temperature (top center)
TB:
Board temperature
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For technical questions, contact: optocoupleranswers@vishay.com
Document Number: 81165
Rev. 1.0, 03-Aug-09
Application Note
Vishay Semiconductors
Thermal Characteristics of Power Phototriac
THERMAL ANALYSIS EXAMPLE
It is important to note that regardless of the package size and
type, the thermal analysis will need to be done to insure a
solid design. Exceeding the thermal parameters can have
detrimental and sometimes even disastrous consequences.
Following is a thermal analysis example of Vishay power
phototriac VO3526. Its thermal resistances are provided in
its datasheet as table 2.
TABLE 4 - INPUT VALUES
QE
0.028
QOT
0.018
W
QNOT
1.25
W
25
°C
TA
θBA
θE-B
149
158
θNOT-B
75
θE-OT
235
θNOT-E
420
θNOT-OT
243
θE-C
161
θOT-C
157
θNOT-C
150
4-layer board
30
130
°C/W
°C/W
TABLE 5 - CALCULATED NODE
TEMPERATURE AND POWER (TA = 25 °c)
To predict the operating temperatures outside of a modeling
environment use the derived resistor network values with
appropriate case-to-ambient and board-to-ambient thermal
resistances as shown in table 3.
TABLE 3 - THERMAL RESISTANCE OF
CASE TO AMBIENT AND BOARD TO
AMBIENT
DERIVED
RESISTANCE
(°C/W)
RESISTANCE
BOARD TYPE
θCA (case-ambient)
2S2P
θBA (board-ambient)
2S2P
30
2S0P
90
2S0P
90
θCA
TABLE 2 - THERMAL RESISTANCE FOR
VO3526 POWER PHOTOTRIAC (°C/W)
θOT-B
2-layer board
W
CALCULATED
VALUES
2 LAYER
BOARD
4 LAYER
BOARD
UNIT
TB
95.3
54.0
°C
TC
92.0
67.7
°C
TE
104.0
70.7
°C
TOT
106.4
73.1
°C
TNOT
144.1
109.1
°C
QNOT-OT
0.1551
0.1480
W
QNOT-E
0.0955
0.0915
W
QOT-E
0.010
0.011
W
QNOT-B
0.651
0.735
W
QOT-B
0.071
0.121
W
QE-B
0.059
0.112
W
QNOT-C
0.35
0.28
W
QOT-C
0.09
0.03
W
QE-C
0.075
0.0183
W
130
Document Number: 81165
Rev. 1.0, 03-Aug-09
For technical questions, contact: optocoupleranswers@vishay.com
www.vishay.com
299
APPLICATION NOTE
It is important to note that the θBA is dependent upon the
material, number of layers, and thickness of the board used.
In our analysis, the phototriacs were mounted on 2 and 4
layer boards with thickness of 4 mm. Obviously, the θBA of
the 4-layer board (2S2P) is much less than the 2-layer board
(2S0P).
Using equations 1 to 14, the thermal resistances shown in
table 2 and 3, and assuming emitter and detectors power
dissipation shown in table 4, the node temperatures when TA
is known are calculated to be as shown in table 5.
Obviously, the majority of heat leaving the package exits
through the lead frame and into the board (QNOT-B), so that
the junction temperatures (TNOT, TOT, TE) when the module
is mounted on the 4-layer board (2S2P) are much lower than
that when on the 2-layer board (2S0P).